Various possibilities are known for realizing an oscillator in integrated semiconductor circuit technology. While, for so-called LC oscillators, the oscillating frequency is set by an inductance and a capacitance, for ring oscillators, this is realized by propagation effects. For a so-called relaxation oscillator, which is also designated as a toggle generator, a capacitor determines the time response. The capacitor is alternately charged and discharged. In this way, the capacitor is normally charged until its voltage exceeds an upper threshold. When this occurs, a discharge process is started and the capacitor is discharged. Then, for example, when the capacitor falls below a lower threshold with its voltage, the entire cycle is repeated. This results in a triangular or sawtooth-shaped voltage profile.
Such an oscillator arrangement according to the class is shown, for example, in FIG. 1 in the document Michael P. Flynn, Sverre U. Lidholm: A 1.2-μm CMOS Current-Controlled Oscillator, IEEE Journal of Solid-State Circuits, Vol. 27, No. 7, July 1992.
In portable or battery-operated applications, it is of great importance to realize switching circuits that operate at very low current levels, in order to save energy. This applies especially for devices that are powered by weak current sources, such as, for example, solar cells, or piezoelectric crystals, which transform mechanical energy into electrical energy.
Oscillators are needed in portable medical devices, such as blood-pressure or pulse measuring devices, devices for administering insulin, and also in automotive applications, such as monitoring systems for tire air pressure. For the latter, the oscillator is used to wake up the measurement system at regular intervals. In all of the described cases, it is desirable that the oscillator consumes the lowest possible energy and that it is also insensitive to temperature fluctuations and power-supply voltage fluctuations.
An RC oscillator is specified in the document U.S. Pat. No. 4,205,279. For generating switch-over thresholds, resistors and bipolar transistors are provided. However, this circuit cannot be built entirely using metal oxide semiconductor or MOS circuit technology. In addition, the current source provided, comprising a P-channel MOS transistor wired as a diode with a connected resistor, is not suitable for especially low energy requirements, and the BIAS source suffers from temperature fluctuations.
The document U.S. Pat. No. 4,714,901 relates to a temperature-compensated complementary metal-isolator semiconductor oscillator. A voltage divider is shown, which is refined for generating a temperature-stable reference voltage. However, this principle is not suitable for generating extremely small BIAS currents.
An oscillator with reduced temperature dependence is shown in the document U.S. Pat. No. 4,868,525. This document takes advantage of the property of achieving partial temperature compensation with resistors having different temperature coefficients. However, complete temperature compensation is not possible.
The document U.S. Pat. No. 4,963,840 shows a delay-controlled relaxation oscillator. Two comparators are used for switching the charging and discharging current. Although this circuit has reduced current consumption through the proposed switching principle, it is not suitable for extremely low current consumption due to the voltage divider for generating a reference level.
The document U.S. Pat. No. 5,461,590 shows an oscillator with a constant current source, which works independently of voltage fluctuations and temperature fluctuations. The operation of this constant current source is explained in more detail in document U.S. Pat. No. 5,315,230. Here, it is provided to charge a reference capacitor, in order to generate a delay that fixes the cycle period of an output signal of the oscillator. However, because very large resistors are needed in order to generate a charging current on the order of one μA, this switching circuit is also not suitable for further reduction of the current consumption.
The circuit in document U.S. Pat. No. 5,604,467 comprises a multivibrator with a temperature-compensated current source and a voltage generator for generating a nearly constant voltage as a function of the temperature. Here, because large resistors are also used for generating small BIAS currents, this principle is not suitable for an especially low current consumption of an oscillator.
An oscillator with stable cycle frequency that is not sensitive to temperature fluctuations is shown in document U.S. Pat. No. 6,020,792. There, a bandgap source is used. Currents, which are proportional to the absolute temperature, and also temperature-constant currents are both generated. This is used for charging a reference capacitor. Due to the bandgap circuit that is used, however, this principle is not suitable for generating power-supply currents of less than 100 nanoamperes.